1. Field of the Invention
The invention relates generally to electrically conductive coatings and more particularly to the vacuum deposition of such coatings on a transparent rigid substrate or on a flexible plastic substrate to be laminated thereto, for use as heated windows, especially in automobiles or aircraft.
2. Description of the Prior Art
U.S. Pat. No. 3,698,946 to Kaspaul et al discloses coatings comprising a first layer of titanium monoxide, a second layer of copper, silver, gold, platinum, or tin, and a third layer of titanium monoxide. The coated articles are useful as photodetectors, light emitting devices, image converters and image amplifiers. While the coated articles are described as transparent and electrically conductive, the transmittances of 38 to 76 percent and resistances of 1600 to 200,000 ohms per square are unsuitable for certain applications such as motor vehicle windows which require a high transmittance, 75 to 80 percent or higher, and very low resistance, preferably less than 10 ohms per square, to develop useful amounts of heat with available generator voltages. In addition, the titanium monoxide imparts a blue color to transmitted light.
U.S. Pat. No. 3,962,488 to Gillery teaches a method for making a colorless, highly transparent coating which also has excellent conductivity, the resistance being less than 10 ohms per square. Gillery discloses that the substitution of titanium dioxide for the monoxide of Kaspaul eliminates the color and transparency problems. However, direct deposition of titanium dioxide is incompatible with the intermediate conductive layer. An intermediate silver film, for example, which is intially continuous and highly conductive becomes discontinuous, resulting in a marked increase in resistance and decrease in transmittance in less than 24 hours. Gillery's invention involves depositing the titanium oxide layers as TiO.sub.x wherein x is greater than 1.0 but less than 2.0. While the coating may intially be somewhat colored, it becomes colorless upon exposure to a normal atmosphere of air or when subjected to the conditions of lamination as the titanium suboxide is oxidized.
Films of titanium suboxide such as described above, made by reactive vacuum evaporation of titanium metal, are extremely active and tend to oxidize extremely rapidly when air is admitted to the vacuum chamber. While such reactivity is desirable, since oxidation to the colorless dioxide is the object of the Gillery invention, the heat produced by such a rapid reaction, as well as the rapid change in volume of the titanium oxide film affect the properties of such film and also the properties of the adjacent electroconductive film. For example, the expansion or heat generated by the oxidation of the titanium suboxide may be sufficient to disrupt the continuity of the adjacent electroconductive film, making it less conductive.